The networking of control units, sensor systems and actuator systems with the aid of a communications system, that is, a bus system, has increased in recent years in the construction of modern motor vehicles and in machine construction, especially in the field of machine tools and in automation. In this context, synergies from the distribution of functions to several control units can be achieved. These are called distributed systems. The communication between various users of such distributed systems is occurring more and more via a bus or a bus system. The communications traffic on the bus system, access and reception mechanisms, as well as error handling are governed via a protocol.
In the automotive field, an established protocol is the CAN (controller area network). This is an event-driven protocol, i.e. protocol activities such as sending a message are initiated by events which have their origin outside the communications system itself. Unique access to the communications system or bus system is solved by a priority-based bit arbitration. The presupposition for this is that a unique priority is assigned to each message. The CAN protocol is very flexible. When using it, the addition of additional nodes and messages is possible without a problem, as long as there are still free priorities (message identifiers).
An alternative approach to such an event-controlled, spontaneous communication is the purely time-controlled approach. All communications activities on the bus are strictly periodic. Protocol activities such as sending a message are triggered only by progress in time valid for the entire bus system. Access to the medium is based on the apportionment of time periods during which a user has an exclusive transmission right. Such a protocol is comparatively inflexible. Adding new nodes is possible when the respective time periods were left free ahead of time. This circumstance makes it necessary already to fix the message sequence before initial operation. Thus, a timetable is drawn up which has to meet the demands of the message requirements with respect to rate of repetition, redundancy, deadlines, etc. The positioning of the messages within the transmission periods must be matched to the applications which produce the message contents, in order to hold the latency between application and point of transmission time to a minimum. If this matching does not take place, the advantage of the time-controlled transmission (minimal latent jitter when sending the message on the bus) is lost. Thus, stringent requirements are placed on the planning tools.
The attempt at an approach, as described in German Patent Applications Nos. 100 00 302, 100 00 303, 100 00 304 and 100 00 305, of the time-controlled CAN, of the so-called TTCAN (time-triggered controller area network) satisfies the requirements outlined above for time-controlled communications, as well as the requirements for a certain degree of flexibility. TTCAN fulfills this by setting up the basic cycle into so-called exclusive time windows for periodic messages of certain communications users and into so-called arbitrating time windows for the spontaneous messages of several communications users.
Besides the bus systems mentioned, a plurality of bus or communications systems for connecting users in distributed systems is available. Subsequently, a starting print is from a TTCAN network as the bus system having at least one user, where this should not be understood as restrictive with respect to the later subject matter of the present invention. Rather, the subject matter of the exemplary embodiments of the present invention described herein may be used also for further comparable bus systems for forming a pulse or clock pulse or pulse division.
In this context, for example, in networked control units in automation, in motor vehicles and in other fields of application, a uniform clock pulse must be derived from the various internal, local clock pulses of the users, especially of the control units for the communications network, that is, the bus system. Likewise, in the networked control units named, an internal, local clock cycle of each user must be derivable, for instance as working clock cycle, from one clock cycle transmitted via the bus system or the communications network.
Customary pulse scalers allow for deriving a slower output clock cycle from an input clock cycle, the clock cycle period of the output clock cycle being an integral multiple of the clock cycle period of the input clock cycle.